There are many applications which require the removal of foreign particulate matter from a process fluid. Among these applications are chemical production processes, water purification and mineral extraction.
The most common process for removing particulate matter from a fluid is that of filtration. In its simplest form the filtration process comprises passing the fluid carrying the particulate matter through a filter material comprising a porous medium. The openings in the filter material allow the fluid to pass therethrough while the particulate matter greater in size than the pores of the filter are stopped. The retention of the particulate matter in the filter removes them from the fluid which has passed therethrough. Many types of filter materials have been used including metal screens, fibrous materials, porous glass and ceramic, paper, sand, glass beads, and certain plastic and chemical powders.
In each filtration process the number of particles entrapped in the filter gradually increases as more fluid passes therethrough and this causes a reduction in the number of open pores available for the passage of fluid. Thus, as the filter becomes clogged there is a reduction in the flow rate of the fluid through the filter. The fluid flow rate is also dependent upon the pressure differential across the filter, the pressure serving to force the fluid through the filter. However, as more particulate matter is collected in the filter, greater pressure is required to maintain a constant flow rate.
When the filters become excessively clogged by particulate matter the rate of fluid flow will be reduced to a low level. The particulate matter must therefore be removed in order to restore the former flow rate. This has conventionally been accomplished by either back flushing the filter to remove the entrapped particles or by replacing the filter with clean filter material. The frequency at which filters must be either back flushed or replaced is dependent upon the flow rate of fluid through the filter and the density of particulate matter within the fluid.
In one type of filter a drum or series of cylindrical discs are covered with a cloth filter and rotated so that the lower portion of the drum or disc is immersed in the fluid to be filtered. The interior of the drum or disc is equipped with a plurality of compartments which are evacuated when the compartment is immersed in the fluid thereby causing the fluid to be drawn through the cloth filter. The particulate matter thus removed is held on the cloth filter. The drum or disc is then rotated out of the fluid and the particulate matter is scraped from the cloth filter. This however is a slow and cumbersome process which can handle at best only low flow rates.
There exists a need for a filter which can remove particles in the sub-micron size range, permit the free flow of fluid through the filter and trap the foreign particles in such a manner that permits continuous removal of the particulate matter from the filter so as to achieve continuous, full flow rate for the fluid.